1. Field of the Invention
This invention relates to an optically resonant structure and has particular but not exclusive application to a vertical cavity surface emitting laser (VCSEL).
2. Related Art
In a conventional Fabry Perot diode semiconductor laser, a resonant cavity is formed extending longitudinally in the plane of the semiconductor substrate with reflective structures at opposite ends. In contrast, in a VCSEL, the resonant cavity is arranged vertically in the substrate with the advantage that light is transmitted perpendicularly from its surface over a larger area than with a horizontal structure, which renders the VCSEL well suited to launching signals into optical telecommunications fibres. An example of a VCSEL is given in IEEE Photonics Technology Letters Vol. 7 No. 6 June 1995 pp 608-610, M. A. Fisher et al. The device consists of a semiconductor substrate on which are formed first and second reflective means that comprise periodic Bragg reflective structures, with a resonant cavity between them that includes a layer of laser active material with spacer layers to provide a sufficient cavity thickness to achieve resonance at a desired operating wavelength .lambda.. The first reflective means may comprise a plurality of interleaved layers of different semiconductor materials with different refractive indices of .lambda./4 effective thickness, overlying the substrate. The Bragg structure of the second reflective means overlies the cavity and is formed of layers of dielectric material, with different refractive indices, of .lambda./4 effective thickness.
In order to pass current through the laser active material in the cavity, a metal contact is formed on the underside of the substrate and a conductive contact layer is formed between the dielectric second reflective means and the layers in the resonant cavity. Current is confined to the cavity by a peripheral reverse-biassed junction.
The thickness of the resonant cavity is typically of the order of 1 .mu.m, whereas in a plane perpendicular to the thickness, the cavity has a relatively large transverse dimension; the cavity is typically square or circular when viewed from above with a diameter or side length of the order of 5.about.20 .mu.m.
A problem that arises in conventional VCSEL structures is that during resonance of the cavity, the transverse mode of resonance is left largely uncontrolled and the relatively large diameter of the cavity may support a number of different transverse modes, which leads to mode competition and instability.
Use of a surface grating of concentric circular elements in a conventional distributed feedback (DFB) or distributed Bragg reflector (DBR) laser, with a horizontal, rather than vertical resonant cavity is disclosed in "Circularly is Symmetric Operation of a Concentric-Circle-Grating, Surface-Emitting, AlGaAs/GaAs Quantum-Well Semiconductor Laser", T. Erdogan et al, Appl. Phys. lett. 60 (16) Apr. 20, 1992 pp 1921-23. With this device, a conventional transversely extending resonant cavity is provided with a surface grating which produces surface emission from the laser by second order Bragg reflection. The circular pattern of the grating results in the emission of a circularly symmetric beam.
Another device is described in U.S. Pat. No. 5,301,201 and "Zone Laser" Appl. Phys. Lett. 65 (2) Jul. 11, 1994 pp 144-146, D. Vakhshoori et al, in which the laser cavity is divided into a plurality of concentric zones which each support an individual resonant mode. The outputs of the concentric zones sum together in a similar way to light from the zones of a Fresnel lens, thereby focusing the laser output into a single spot.
In Applied Physics Letters, Vol 66, No. 21, May 22, 1995, pp 2769-2771, J. H. Ser et al, there is described a VCSEL which has an overlaid fine metal interlaced grating etched on its upper DBR, overlying its light emitting face. The purpose of the grating is to produce polarisation stabilisation, for example to select TM polarisation as the dominant polarisation mode.